1. Field of the Invention
The present invention relates to a wet type friction material used in a friction engaging device such as a clutch and a brake employed within oil in an automatic transmission of a motor vehicle.
2. Related Background Art
A wet type friction material has been used in an automatic transmission of an automatic motor vehicle and the like. The automatic transmission of the automatic motor vehicle includes a multi-plate clutch in which a plurality of friction plates each formed by sticking wet type friction materials to surfaces of a metallic substrate (core plate) and a plurality of separator plates as friction counterpart materials each formed by a single plate such as a metallic plate are alternately arranged and is designed so that a driving force is transmitted by abutting these plates against each other or interrupting by releasing these plates from each other within ATF (automatic transmission fluid) used as a lubricating oil.
As a wet type friction material used in the oil in this way, a wet type friction material of paper type called as “paper friction material” is generally used. This wet type friction material is generally formed by paper-making fiber base materials such as natural pulp fibers, organic synthetic fibers or inorganic fibers with filler such as diatom earth or cashew resin and friction regulator in a wet manner and then by impregnating binder comprised of thermosetting resin and then by curing with heat. As the binder resin material, phenol resin having excellent heat-resistance, high mechanical strength and good compression fatigue property has conventionally been used mainly. Such a prior art is disclosed in Japanese Patent Application Laid-open No. 7-197016.
By the way, in a recent industrial field regarding automobiles, light weight and high efficiency of various parts have been sought in order to save energy and to achieve light weight. On the other hand, there is a tendency for seeking high rotation and high output of an automobile engine. Also in an automatic transmission, in order to cope with the compactness and light weight of the friction engaging device and the high rotation and high output of the automobile engine, regarding the wet type friction material, enhancement of coefficient of friction and further improvement in heat-resistance and endurance have been sought.
In order to resolve these problems, a technique for compounding high hard filler such as alumina to the paper base material has been investigated. However, in this technique, although coefficient of friction is great in an initial engaging condition, since an attacking force against the friction counterpart material is strong, as the usage of the friction material is advanced, the coefficient of friction is gradually decreased. Thus, this technique is not satisfactory. Further, in the past, resins other than phenol resin used as the binder, for example, silicone resin has also been investigated (for example, refer to Japanese Patent Application Laid-open No. 7-197016). The silicone resin has excellent heat-resistance and endurance based on siloxane bonding of main structure and does not generate a burned area called as “heat spot” on a friction counterpart material (separator plate) under a high temperature/high load condition and has excellent long term stability of coefficient of friction. However, since the silicone resin is soft, a compression fatigue property thereof is bad and a displacement amount of a total thickness of the friction material becomes great. Thus, a satisfactory friction material could not be obtained. Further, a μ−V (coefficient of dynamic friction−velocity) property may have negative gradient, with the result that, the friction material is used in a clutch, transmission shock may occur.
That is to say, regarding the wet type friction material, there is a need for seeking further improvement in three factors, i.e. high coefficient of friction, heat resistance and positive gradient of μ−V property.